The present invention relates to a semiconductor light emitting device and a method for manufacturing the same.
In recent years, among semiconductor light emitting devices, Light Emitting Diodes (LEDs) are widely used in optical communications, information display panels, auxiliary light sources for CCD cameras, LCD back lights and the like. It is important for these light emitting diodes to have high luminance. The luminance of the light emitting diodes is dependent on a ratio of the number of passing electrons to the number of photons emitted outside, i.e., external quantum efficiency. The external quantum efficiency is determined by internal quantum efficiency, that is a ratio of passing electrons to photons produced in emitter layers of the light emitting diodes, and by an external emission efficiency, that is a rate of the produced photons which is emitted out of the devices. Among them, the external emission efficiency is largely influenced by device structures.
In the light emitting diodes, it is effective for the purpose of improving the external emission efficiency to use translucent substrates transparent to emission wavelengths. This is because when substrates opaque to emission wavelengths are used, light is emitted only from the upper surface, whereas when the transparent substrates are used, light can be extracted not only from the upper surface but also from the side surfaces. Moreover, light reflected off the lower surface may be emitted from the upper surface and the side surfaces. This method is applied to infrared light emitting diodes using InGaAsP-based semiconductor materials, red and infrared light emitting diodes using AlGaAs-based semiconductor materials, yellow light emitting diodes using GaAsP-based semiconductor materials, green light emitting diodes using GaP-based semiconductor materials and the like.
In the above-stated methods for manufacturing a semiconductor light emitting device, the GaP transparent substrate is directly bonded to an intermediate layer of the emitter layer or the GaP adhesion layer provided on the intermediate layer of the emitter layer. For the direct bonding, it is important to smooth the junction surfaces so that the both surfaces are closely fit to each other. However, as the intermediate layer of the emitter layer is thin, it is not possible to smooth both surfaces by polishing or other techniques if hillocks are generated. Moreover, the GaP adhesion layer is not matched to the emitter layer, and therefore not only its surface fails to be a smooth mirror surface, but also hillocks, which are protruding-type crystal defects, tend to be generated. Once hillocks are generated, it is difficult to obtain a completely smooth surface even if polishing is applied, and therefore direct bonding is not achieved in the vicinity of the hillocks and so voids are generated, causing the yields to be decreased.
The GaP transparent substrate 82 is not matched to the AlGaInP intermediate layer 84, and therefore it is not possible to form the emitter layer 83 on the transparent substrate 82 with good crystallinity by normal layer growth method. Accordingly, several methods for manufacturing semiconductor light emitting devices with use of transparent substrates that are not matched to emitter layers have been proposed and disclosed in the following patent documents.
Patent Document 1: Japanese unexamined patent application No. H06-302857
Patent Document 2: Japanese unexamined patent application No. H06-296040
Patent Document 3: Japanese unexamined patent application No. 2000-196139
Patent Document 4: Japanese unexamined patent application No. 2001-144322
In the Patent Document 1, disclosed is a method for manufacturing a semiconductor light emitting device, composed of the steps of: forming an emitter layer on a GaAs substrate opaque to emission wavelengths by epitaxial growth method; forming a GaP current diffusion layer of several dozen μm on the emitter layer by epitaxial growth method; removing the GaAs substrate opaque to emission wavelengths; placing a GaP transparent substrate on the plane from which the GaAs substrate is removed; and applying heat treatment so as to directly bond the GaP transparent substrate. The thickness of the GaP current diffusion layer formed on the emitter layer is about 50 to 100 μm in view of a growth time and the mechanical strength of the wafer after removal of the GaAs substrate. This is because if the thickness is 50 μm or smaller, the wafer tends to break down when being treated, whereas if the thickness is 100 μm or larger, a growth time becomes longer and the costs of the light emitting diodes become higher.
In the patent document 2, disclosed is a method for manufacturing a semiconductor light emitting device, composed of the steps of: forming an emitter layer on a GaAs substrate opaque to emission wavelengths by epitaxial growth method; placing a GaP transparent substrate transparent to emission wavelengths on the emitter layer and applying heat treatment so as to directly bonding the GaP transparent substrate; and then removing the GaAs substrate opaque to emission wavelengths.
Further, in the patent document 3, disclosed is a method for manufacturing a semiconductor light emitting device, composed of the steps of: forming a p-type semiconductor layer that functions as a current diffusion layer on an undoped or n-type GaAs substrate; forming an emitter layer thereon; forming an n-type GaP adhesion layer thereon; applying heat treatment so as to bond the n-type GaP adhesion layer and an n-type GaP transparent substrate; and then removing the GaAs substrate.
Further, in the patent document 4, disclosed is a method for manufacturing a semiconductor light emitting device, composed of the steps of: forming an n-type GaP adhesion layer by Metal-Organic Chemical Vapor Deposition (MOCVD) method on an emitter layer; and applying heat treatment so as to bond the n-type GaP adhesion layer and an n-type GaP transparent substrate.
In the above-stated methods for manufacturing a semiconductor light emitting device, the GaP transparent substrate is directly bonded to an intermediate layer of the emitter layer or the GaP adhesion layer provided on the intermediate layer of the emitter layer. For the direct bonding, it is important to smooth the junction surfaces so that the both surfaces are closely fit to each other. However, as the intermediate layer of the emitter layer is thin, it is not possible to smooth the both surfaces by polishing or other techniques if hillocks are generated. Moreover, the GaP adhesion layer is not matched to the emitter layer, and therefore not only its surface fails to be a smooth mirror surface, but also hillocks, which are protruding-type crystal defects, tend to be generated. Once hillocks are generated, it is difficult to obtain a completely smooth surface even if polishing is applied, and therefore direct bonding is not achieved in the vicinity of the hillocks and so voids are generated, causing the yields to be decreased.